The development of mobile telephony has created a need for simple and inexpensive mass production of antennas for, inter alia, base stations. In order to achieve better and more reliable coverage, which is of particular importance within mobile telephony, use is advantageously made of polarization diversity. Two different polarizations, usually at right angles to one another, are used, preferably via one and the same antenna. Mobile telephony utilizes frequency ranges which lie within the microwave range, for which reason an antenna which is often used is the microstrip antenna. Like some other microwave antennas, microstrip antennas comprise radiating antenna elements which are mounted in front of a ground plane. Dual-polarized antennas commonly found within, for example, mobile telephony are polarized using 0/90.degree. or .+-.45.degree.. Dual-polarized antennas exist in two basic variants, the first of which comprises two different types of single-polarized antenna elements in order to produce two different polarizations, and the second comprises dual-polarized antenna elements instead. Both variants of dual-polarized antennas have advantages and disadvantages. A requirement has arisen, in particular within mobile telephony, for antennas with .+-.45.degree. polarization because this type of polarization appears for the time being to have more advantages, such as a more symmetrical propagation/attenuation, compared with 0/90.degree. polarization. The present invention concerns the problems which arise with regard to single-polarized antenna elements in the form of patches above a ground plane in, for example, said types of dual-polarized antennas. The patches may be, for example, aperture-fed or probe-fed.
It is important that the two polarizations in dual-polarized antennas have antenna patterns which are as equivalent as possible and that every polarization vector maintains its direction for all azimuth angles. In order to achieve this, it is important, inter alia, that each linear-polarized antenna element, patch, is excited in only one polarization, that is to say that the cross-polarization must be kept as low as possible. A reduction of the cross-polarization in a patch can be achieved by, for example, decreasing the width of the patches. If the width of the patches is decreased, the bandwidth is also decreased and the lobe width increases.
Moreover, it is also important that the coupling between the patches for the two polarizations is as low as possible. One solution in this respect could be to increase the distance between the patches, but if the distance between patches for the same polarization is increased, problems arise with undesirable grating lobes which impair the applicability of the antenna. A dual-polarized antenna with single-polarized rectangular patches is a construction in which traditionally a compromise between bandwidth and cross-polarization and also between coupling between patches and grating lobes has been made.